Colorado is one of few places that is blessed with all four seasons distinctly. Unfortunately for farmers and gardeners, this means a shorter growing season when the snow and frost sets in and sends plants to their deaths or into hibernation for the winter.
Greenhouses were invented to extend the growing season in colder climates. The idea is that the walls and roof allow the sun in and keep the interior warm and humid even if those aren’t the conditions outside. There are many different varieties of greenhouses that all accomplish this insulating task.
At Speedwell Farm and Gardens in Longmont, they have what are known as tunnel gardens or hoop houses. Instead of a solid structure, there are rings creating a tunnel over the plants. Sheets of material, known as glazing material, are put over the rings and can be rolled up when the weather is nice and rolled down when it gets cold.
The curve of the structure is utilized to the fullest by putting tomatoes, a taller plant, in the middle where there is the most room vertically. On the sides are rows of shorter pepper plants and mixed in with the peppers are flowers that work as a natural deterrents to pests.
When the weather gets cold the plastic is rolled down and the interior is heated with propane. To reduce, and in some cases even eliminate, the need for additional, external heat, underground greenhouses are built. Underground greenhouses are known by many names. Pit greenhouses, trench greenhouses, and walipini are a few.
Underground greenhouses are known by many names. Pit greenhouses, trench greenhouses, and walipini are a few.
An underground greenhouse is similar to a hoop house except instead of being constructed fully above ground, it is dug about six to eight feet down and then covered with glazing material. The purpose of this is to maximize the greenhouse’s efficiency by having more of the encompassing area be stone, clay, or dirt, materials that absorb and hold heat at temperatures from 50 to 60 degrees Fahrenheit. In an above ground greenhouse, only the floor absorbs heat throughout the day and then helps keep the area warm at night. In an underground greenhouse the floor and four walls absorb and redistribute heat so the plants are being warmed from five sides instead of one. Since the walls and floor do more of the work, less energy is required to maintain heat in the space.
In addition to having underground hoop houses, Harlequin’s Gardens in Boulder has what they call their A-Frame greenhouse. It is dug into the ground but instead of a hooped roof and glazing material it has glass panels on the south-facing side and insulation on the north side that meet at a point.
Most heat escapes a greenhouse through the glazing material, so this A-Frame structure is more efficient in two ways. The insulation on the north side allows the greenhouse to retain more heat and allows less to escape at the same time. It might sound redundant, but it is actually twofold. The sun is most directly overhead in the summer when less heat needs to be retained anyway, so any that doesn’t enter through the north side is not missed. During the winter the sun is at an angle and only comes in through the south side anyway.
Another thing they do to help retain heat is put large barrels of water in their greenhouses. These barrels function the same way the ground does in that it absorbs heat from the sun during the day and redistributes it back into the air at night.
At Harlequin’s Gardens, their underground greenhouses are so effective that they don’t need to use a supplemental heat source like the ones Speedwell Farm and Gardens’ do. In fact, shading material is used when the weather is warm to help prevent overheating. If the weather really dips far below freezing, then they occasionally put a space heater in overnight just to be safe, but they have no need for propane to heat their greenhouses regularly. Mikl Brawner, the owner of Harlequin’s Gardens alongside Eve Reshetnik Brawner, said, “I have actually grown tomatoes in that house, in the winter, down to zero degrees with no supplemental heat.”
“I have actually grown tomatoes in that house, in the winter, down to zero degrees with no supplemental heat.”
When Mikl Brawner built Harlequin’s first greenhouse in 1991 it was very expensive up front, as is typically the case with sustainable energy projects. In the decades since its construction it has “had practically no maintenance costs.” Between the lack of energy needed to heat it and low maintenance costs it was a good financial investment. Even more importantly though it means the greenhouse has a low carbon footprint.
Being environmentally sound is very important at Harlequin’s Gardens. Not even weeds are wasted. They are fed to the farm’s variety of resident ducks.
If you have the space for an underground greenhouse and are willing to take on a bit of a project, it is possible to build one yourself. The Benson Agriculture and Food Institute put together a handbook on how to build a walipini and they can walk you through the details. Some of the bigger considerations are location, frame materials, and glazing materials.
Since Colorado has such hard, rocky soil (which earned Boulder its name), digging a pit by hand would be a major undertaking. To make life easier, backhoes can be rented. Prices will vary depending on factors like where you are digging and how long the project takes. Another potential cost is if you choose to hire an operator or not. It’s easy to say you can do it yourself, but unless you actually have experience operating one it’s probably wiser to leave it to a professional.
Once you have your hole, a frame will need to be built over it. The materials can range in price, durability, and ability to insulate so you’ll want to take a moment to look at all of your options. Advance Greenhouses offers an assortment of greenhouse kits as well as greenhouse glazing materials and explanations about their differences. If you choose to order from them, all of their materials are cut to order so you know you’ll get exactly what you need.
Advanced Greenhouses Material
If you have the space and want your own pit greenhouse but don’t think a massive DIY (Do It Yourself) project sounds like fun, don’t worry. There are other options. Earth Love Gardens can install the frame and glazing material over a hole that you had dug beforehand eliminating a lot of the work for you.
If an underground greenhouse isn’t in the cards for you, Earth Love Garden can also install a garden bed or aboveground hoop house for you. These alternatives aren’t as energy-efficient as a walipini, but they do extend the growing season.
Whatever type of garden you may choose, there are additional steps you can take to increase its sustainability. Sustainable Village focuses on irrigation and their Blumat watering system has analog moisture-sensing valves that use pressure from the moisture in the soil to know when to release water. Low levels of moisture in the soil cause it to un-pinch and allow water to flow. Once the soil is adequately moist, the pressure rises and the tube pinches again causing water flow to cease. Since it is an analog system there is no electric energy needed to run it. It is like a drip irrigation system except it isn’t on a timer or running all of the time since it can react to the soil’s moisture level.
Because these systems regulate themselves there is no fear of over or under-watering. They can be calibrated to allow more or less water depending on what you are trying to grow and that plant’s water needs.
Another irrigation product Sustainable Village offers is their low pressure soaker hoses. The low pressure soaker hoses are mats that essentially sweat out water slowly into the soil. These can be on timers or they can be used with a Blumat system.
Sustainable Village can design an irrigation system for you based on your garden and your watering needs. They don’t install the systems, but all you’ll need to do it yourself is a pair of scissors.
Going one step further with sustainability, these systems could also be hooked up to rain barrels to collect and reuse precipitation, making them both energy and water-efficient.
At this point, an underground greenhouse probably sounds like it is either a whole lot of money, a whole lot of work, or even both. So why bother with getting an underground greenhouse?
“Spending more money upfront saves a lot of money in the long run,” as Mikl Brawner says. Underground greenhouses are an investment that pay off both financially and environmentally long term. The most obvious perk of any kind of greenhouse is that it extends the growing season and therefore the harvest. If you can do that and do it while being environmentally conscious then you really have something to be proud of.
“Spending more money upfront saves a lot of money in the long run,”